1. field of the Invention
The present invention pertains, in general, to a seal assembly for pistons and the like. More particularly, it relates to a novel and improved three-piece seal assembly especially adapted for use in sealing large clearances existing in piston-cylinder assemblies and which essentially includes a novel and improved elastomeric sealing ring and a pair of novel and improved bearing members associated and cooperating with opposite sides of the sealing ring.
2. Description of the Prior Art
In industry, it is rather conventional practice to protect against uncontrolled fluidic leakage through the application of seals and seal assemblies. One particular type of application in which the prevention of undesired leakage of fluid is highly desirable is in standard fluid cylinders, especially in high pressure situations. A rather typical type of fluid basically includes a piston cylinder wall and slidably disposed therein a movable piston. The piston usually has a seal or seal assembly mounted on the external periphery thereof. Normally fluidized pressure is introduced into the cylinder for purposes of advancing the piston. The seal serves to enhance such advancement by substantially blocking passage of all but a small amount of fluid which would otherwise have a tendency to pass around the periphery of the piston. Thereby, a more efficient utilization of fluid power is obtained. Additionally, to further minimize the tendency of unwanted fluid leakage about the piston, it is customary to have the corresponding piston cylinder walls rather accurately machined within close tolerances of the piston. In this particular manner, through the close radial tolerances, there is less likelihood that unwanted leakage will occur.
Heretofore, in the prior art there have been many kinds of piston seals and seal assemblies which are constructed and arranged in a wide variety of ways and from different types of materials.
Since the seal is crucial in the successful functioning of fluid cylinders, it is extremely important that material characteristics, such as hardness, modulus of the material, abrasion resistance, tear strength and compatibility with various materials be selected in conjunction with the particular design configuration of the seal to achieve such successful operation. In general, while in given circumstances certain seal assemblies perform rather adequately, there are, however, many situations wherein such assemblies suffer serious disadvantages. One of the primary disadvantages ordinarily associated with the use of many of these known types of sealing rings, especially under high fluid pressure applications, is extrusion. Extrusion normally occurs whenever the O-ring or other sealing ring is forced between the rather small radial clearance or tolerance existing between the piston and the internal peripheral wall portion of the corresponding cylinder. As a consequence, the material of the sealing ring has a tendency to be extruded and will be abraded or sheared off during the normal relative movement between the piston and the cylinder wall. It will be appreciated in those circumstances where the radial clearance is larger than normal, the tendency for extrusion is correspondingly increased. In such event, the seal tends to be rendered unsuitable for the purposes intended.
Another similar disadvantage which frequently arises in the field is the fact that the sealing elements or O-rings and the like have the tendency to roll or twist. Rolling or twisting tends to cause these sealing elements or O-rings to split and otherwise break. It is evident that such results will likewise render them unsuitable for the purposes intended.
Still another shortcoming in this field is the somewhat commmon practice to machine the internal peripheral cylinder wall to a close tolerance with respect to the piston. Since in commercial practice such tolerances must be rather accurately machined, such, of course, necessarily results in the undesirable feature of increased manufacturing costs. In an effort to overcome this particular cost problem, manufacturers of fluid cylinders and seals have tended to machine cylinder walls so that they have relatively large tolerances with respect to the piston. At this point, it should be emphasized that the field of fluid cylinders, seals and sealing assemblies is extremely competitive and there is somewhat of a narrow or small profit margin associated with the sales of such fluid cylinder seals and sealing assemblies. As a consequence thereof, it will, of course, be appreciated that even slight increases in costs may render such fluid cylinders and corresponding seals and sealing assemblies commercially less desirable than competing cylinders, seals and sealing assemblies. In addition, as earlier mentioned with greater tolerances or clearance the likelihood for extrusion of the sealing element as well as rolling and twisting of the same is increased. Accordingly, many conventional sealing assemblies were found to be unsuitable for use in those situations wherein larger tolerances existed especially at high pressures. In particular, under the foregoing conditions, these known seals and sealing assemblies do not possess adequate relationships between high shear modulus, high abrasion resistance, high tear strength and compatability with many fluids to successfully operate as well as be economical to manufacture. Additionally, traditional design configurations for primary sealing members have, under the above circumstances, in general, been unsatisfactory. Moreover, such seals and sealing assemblies are either complicated in construction and/or relatively expensive to produce.
Conventional O-rings and the like which are fabricated from rubber are not found to exhibit suitable wear, tear, and abrasion resistance characteristics as well as possess high modulus of material for ordinary temperature ranges at particular hardnesses, especially under high fluid pressure applications wherein relatively large clearances exist between the piston and corresponding cylinder wall. As a result, such O-rings and the like have a relatively short seal life. Therefore, frequent maintenance and replacement are required. Aside from these deficiencies, the conventional rubber O-rings suffer from other shortcomings in that they are normally manufactured through a compression molding process. Compression molding produces sealing elements that have seams. Such seams must be relatively smooth and continuous. This unnecessarily adds to manufacturing cost factors. Additionally, conventional rubber O-rings are generally incapable of being fabricated such that they can advantageously accommodate for large extrusion gaps since they do not possess adequate strength and performance characteristics.
Besides rubber, there are other heretofore known materials which have been employed to form sealing elements. Teflon, Kel-F and Nylon are examples of such sealing materials. These other materials, however, have been found not to be satisfactory, particularly from the standpoint of having a reliable, long-wearing sealing ring used in applications having relatively large clearances and high fluidic pressures. Moreover, both Teflon and Kel-F are the kinds of materials which are basically non-elastomeric as well as are thermoplastic. Additionally, this latter group of materials is subject to cold flow problems which renders them less effective for sealing purposes. Nylon is also relatively unsuitable for sealing purposes since it is basically a rigid plastic of the thermoplastic type. Polyurethane has been previously used as a material for certain types of seal rings, but it has not been used other than as a relatively conventional sized and shaped O-ring or other conventional sized and shaped seal.
Furthermore, several known prior art sealing constructional arrangements for pistons suffer from the disadvantages of being extremely complex and expensive to manufacture. By way of specific example, one particular type of known sealingassembly is comprised of five separate and distinct components. Included in this assembly is an oversized rubber sealing element having an enlarged cross-sectional area, a pair of disc-shaped back-up rings on opposite sides of such sealing element, and a pair of separate bearing rings, each disposed adjacent a respective back-up ring. Significant disadvantages are associated with this arrangement since with the large number of components, such assembly is expensive to produce and complicated in construction. Additionally, such back-up rings are unable to prevent both extrusion and rolling. Moreover, the primary sealing element is fabricated from rubber and is subject to the numerous aforementioned shortcomings associated with rubber O-rings and the like.
There have been other seal assemblies proposed which reduce cost by reducing the total number of sealing components. An example of such type of sealing assembly is a three-piece arrangement generally referred to as a Simko seal. This particular sealing arrangement includes two bearing rings and a composite seal member. Such a seal assembly, however, in order to satisfactorily perform must have extremely close and fine metal tolerances and, therefore, quite expensive surface finishes. Of course, such arrangement is expensive to produce. Additionally, the sealing element is expensive to produce, since it requires an embedded elastomeric seal element surrounded by a special rubber and fabric composite casing. It will be recognized, of course, that not only is this composite rubber seal member subject to the disadvantages previously mentioned in regard to rubber sealing members, but also, this kind of composite seal construction is expensive to produce and machine finish. Moreover it can only be used where there are close clearances or tolerances.
Another heretofore known known of three-piece seal is basically disclosed in U.S. Pat. No. 2,728,620. In the above-referenced patent, a conventional O-ring is used in conjunction with a pair of back-up rings. Drawbacks associated with this arragement are that the conventional O-ring is subject to the disadvantages previously enumerated with respect to elastomeric rubber sealing elements, and the back-up rings fail to provide adequate bearing support surfaces for the entire seal assembly.
Still another type of known three-piece construction for a piston seal is manufactured by Dowty in Great Britain. Such seal assembly essentially includes a pair of back-up rings and a rubber primary seal of special nitrile material. Such nitrile material is expensive to manufacture and must include a filler to acquire high extrusion resistance. Moreover, this material is generally stiff and has low elongation. Similarly, as with the latter described three-piece sealing arrangement, the back-up rings do not provide for bearing supports. As a consequence, this particular arrangement as well as the others fails to furnish a seal assembly of simple, economical and reliable construction which can be successfully utilized in high pressure application with large clearances, particularly those in excess of the normal ranges.